1. Field of the Invention
The present invention relates generally to control systems for data processing systems and more specifically to a system for extending the interior decor of the control unit of a microprogrammed computer.
2. Description of the Prior Art
In microprogrammed computers, execution of a so-called macroinstruction, of the type usually written by a programmer in a high-level language (such as FORTRAN or COBOL), requires the performing of a number of machine instructions in a predetermined order. Each machine instruction, in turn, is executed by means of a sequence of microinstructions which are contained in a microprogramming memory (which generally is a read-only memory).
This type of computer organization has the drawback that the machine instructions provided for the computer are of limited number and are strictly correlated to a predetermined set of microinstructions contained in the microprogramming memory. The set of machine instructions which may be performed by means of a set of microinstructions contained in the microprogramming memory constitutes a so-called "Interior Decor" of the computer. Adding or modifying the microinstructions requires addition or the modification to the microprogramming (read-only) memory which records them. Thus, it is difficult and burdensome to extend the "Interior Decor" of a microprogrammed computer.
In the U.S. patent application Ser. No. 508,970 filed Sep. 25, 1974 by the applicant, a microprogrammed computer has been described in which it is possible to recall microinstructions both from a microprogramming memory and from main or working memory of the computer. This allows the addition of new microinstructions for carrying out machine instructions not previously recognized. However, this does not entirely solve the problem because it is still necessary to correlate each machine instruction with an appropriate microprogram (i.e. a set of microinstructions) for its execution.
It is known that a machine instruction consists of a set of information, including an operating code and the addresses of operands. The operating (or function) code, which is different for each machine instruction, specifies an operation to be performed. The operating code, by means of decoding systems, supplies address of the microprogram for carrying out the machine instruction. Therefore, addition of new machine instructions requires change to or addition of new decoding systems for supplying the addresses of new microprograms for carrying out the additional machine instructions.